作者tanaka0826 (田中鬪莉王)
看板Math
標題[微積] 線積分與面積分等問題
時間Mon Aug 29 02:24:27 2011
一次問這麼多題實在不恰當
不過時間迫在眉睫,好幾題算到一半卡住,希望板友們能稍微解釋一下我的問題所在:
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Q1.Find the moment of inertia about the Z-axis of a thin shell of constant
density δ(δ > 0) cut from the cone 4 x^2 + 4 y^2 - z^2 = 0 , z >= 0 ,by
the circular cylinder x^2 + y^2 = 2x
這題要怎麼去計算我大概清楚,但是在找出parametrization(即換成u,v)時找不出適當
的置換方法。
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Q2: Find the area of the surfaces:
Let S be the portion of the cone z = √(x^2 + y^2) that lies over the region
between the circle x^2 + y^2 = 1 and the ellipse 9 (x^2) + 4 (y^2) = 36
in the xy-plane.
跟上題不同的是,我求出了dA = √2 du dv ,但是後面積分的邊界我一時想不出該怎麼
表示出來,因為又有橢圓又有圓,我就卡住了...
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Q3:Integrate g(x,y,z) = x√(y^2+4) over the surface cut from the parabolic
cylinder y^2 + 4z = 16 by the planes x = 0 , x = 1 and z = 0.
跟Q2類似,我把r(u,v)設為( u , v , 4 - (v^2)/4 )然後代入g裡面,以及求出dA,不幸
的是,算出來的答案跟解答不同,正解是56/3。
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Q4:A wire of density δ(x,y,z) = 15√(y+2) lies along the curve
r(t) = (0,t^2-1,2t),-1 <= t <= 1.Find its center of mass.
這題我目測看出X和Z都是0,但是Y怎麼算都算不出-3/5,所以我想確定一下Ym的算法,
該怎麼下手呢?
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Q5:Find the center of mass and the moments of inertia about the coordinate
axes of a thin wire lying along the curve r(t) = ( t,[2√(2t^3)]/3,(t^2)/2 ),
0 <= t <= 2 , if the density is 1
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1+t
這題感覺跟上一題很像,但是我不知怎麼下手...
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真的很抱歉一次問五題,在考題轟炸之下已經有點昏頭了,希望板上高手能幫忙我Orz
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◆ From: 122.117.20.246
推 ntust661 :1.一定要換成 u v 嗎 ? 08/29 14:20
→ ntust661 :2.上下限好找阿XDD 把圓錐畫出來 08/29 14:22
→ ntust661 :然後底部投影到 x - y 平面是圓與橢圓 08/29 14:24
→ ntust661 :那積分由 dz dx dy 開始 08/29 14:24
推 ntust661 :更正,是算表面積的話 08/29 14:27
→ ntust661 :算出法向量為 n = ▽z 08/29 14:28
→ ntust661 :表面積 = dA = │▽f│/│▽f‧k│dx dy 08/29 14:29
→ ntust661 :法向量算法 z^2 = x^2 + y^2 , f = x^2 + y^2 - z^2 08/29 14:30
→ ntust661 :▽f = 2x i + 2y j - 2z k 08/29 14:30
→ tanaka0826 :1.不一定,不過我想不出其他算法 08/29 15:04
→ tanaka0826 :2.就是在這個步驟之後就寫不出來了... 08/29 15:04